1. Field of the Invention
The present invention relates to a power steering device for an automobile, in which the steering is assisted by a power source such as an electric motor.
2. Description of Related Art
As an example of a power steering device of this kind, JP-B2-3-11943 discloses a control system for a power steering device assisted by an electric motor. This control system includes a phase compensator that generates signals proportional to differentiated values of outputs from a torque sensor detecting a steering shaft torque. The phase compensator compensates a delay of a transfer characteristic in a control system caused by frictions and other factors in the steering system. In other words, the phase compensator stabilizes operation of the control system and cancels a delay of a power-assist electric motor due to its inertia by changing a phase of the torque signal. The phase compensator is composed of analog or digital circuits, and its frequency-responsive characteristic functions to increase the gain and advance the phase of the torque signals.
In the control system disclosed in the above publication, however, the torque signals compensated by the phase compensator tend to include noise because the differentiated values of the torque signals are used. The noise is amplified in a frequency region where the gain of the phase compensator is increased, and the amplified noise is included in current for driving the power-assist motor. Therefore, the noise is included in an output of the power-assist motor. As a result, the motor output that includes the amplified noise generates a trembling vibration on a steering wheel, which is felt by a driver especially when a steering wheel is stably held or slowly rotated. If the gain of the phase compensator is lowered to avoid the uncomfortable vibrations, a quick response of the steering system is sacrificed because the phase advance is also lowered. As a result, the power-assist motor does not respond quickly enough to sufficiently assist a steep steering, and the steering wheel becomes heavy, giving uncomfortable steer-feeling to a driver.